Ultra high frequency side band filter network



Jan. 2, 1951 H. KIHN ULTRA HIGH FREQUENCY SIDE BAND FILTER NETWORK FiledApril 26, 1947 Svcs (Ittorneg Patented `an. Z, 1951 ULTRA HIGH FREQUENCYSIDE BAND FILTER NETWORK Harry Kihn, Lawrenceville, N. J., assignor toRadio Corporation of America, a corporation of Delaware ApplicationAprii 26, 1947, Serial No. 744,128 (ci` riss-444) v 11 Claims.

This invention relates generally to ultra-high frequency apparatus andmore particularly to an improved` ultra-high-frequency band-pass lternetwork having relatively wide-band frequency response and sharpfrequency cutoff characteristics.

, In ultra-high-frequency radar and object locator systems employingfrequency-modulated signals, it is customary to utilize a portion of thefrequency-modulated energy derived from the transmitter1 for combinationwith the received reflected frequency-modulated signals derived from theobject to be located. In systems of this type and in vestigial sidebandultra-high-frequency .television receivers, and the like, it isessential that a relatively wide-band, sharp outoif ultra high frequencyfilter network be employed for separating the desired from the undesiredsignal components.

In frequency modulation radar systems of the type described, a portionof the frequency-modu lated transmitter signals is mixed with locallygenerated substantially lower frequency local oscillations to provide asingle side-band signal which may be subsequently mixed with thereceived frequency-modulated signals to provide a substantially lowerintermediate frequency-modulated signal which is detected vand utilizedto provide the desired information with respect to the object to belocated. In a typical system of this type, thefrequency modulated radartransmitter provides frequency-modulated signals having a mean frequencyof 515 mc. A portion or" the transmitter energy is combined in a localoscillator-modulator circuit providing 30 mc. local oscillations. Themodulation components of 30, 435, 515 and 5,45 mc. are applied to thenovel side-band filter network comprising the instant invention, and the30, 515 and 545 mc. signal components are rejected, while the 485 me.frequency modulated signals are transmitted with minimum attenuation toprovide a balanced output frequency-modulated signal having a meanfrequency of 485 mc. which is applied to the balanced first detector ofthe radar receiver. The received frequency-modulated 515 mc. signalsderived from the object to be located are combined in the balanceddetector with the frequency-modulated signals having a mean fre# quencyof 485 mc. derived from the lter network to provide frequency-modulatedintermediate frequency signals having a mean frequency of 30 mc. Theintermediate frequency-modulated signals thence are applied to a secondde tector wherein they are combined with the locally generated 30 me.oscillations, and thence are amplified and utilized to derive therequired in.- formation concerningthe objects to be located.

It is essential that the side band lter network be adjustable both as toits cut-off frequencies and its coupling factors to provide the desiredselectivity. and transmission characteristics. According to the instantinvention, a side band Filter, having a mean pass frequency of 485 mc.and substantially unattenuated transmission sigf-` nal frequenciesbetween 473 and 492 mc. com; prises a hollow shieldingenclosurecontaining a plurality of cylindricah or otherwise shaped, inductiveelements each having an adjustable series capacitor element and a fixedor adjustable par-` allel capacitor element. The inductive elements arearranged in spaced paralleladjustable relation within the shieldingenclosure and, if desired, adjustable coupling shields may be interposedbetween the several inductive elements. Inputsignals are capacitivelycoupled to one of the inductive elements adjacent to one end of theshielding enclosure, and a balanced output circuit is coupled'to twoother ones of the inductive elements located adjacent to the oppositeend'of the shielding enclosure. Ina first embodiment of theinventionthebalanced output circuit may comprise separate connections to each of thetwo output inductors. In a second embodiment of the invention, Vthebalanced output circuit may comprise a coupling loop coupled in phase'oppo sition to the two output filter inductors. In both embodiments ofthe' invention, the several filter stages are each tuned to the desiredmid-frequency of the pass band and the two output lter sections aretuned in push-pull to the desired pass band mid-frequency. The push-pushresonance of the balanced output circuit is selected to occur at afrequency well removed from the desired pass-band. Among the objects ofthe invention are to provide an improved method of and means fornltering ultra-highfrequency signals. Another object is to provide animproved ultra-highfrequency band-pass lter for frequency-modulatedsignals. -An additional object is to provide an improvedultra-high-frequency lter having substantially uniform transmissioncharacteristics over a relatively wide frequency band and relativelysharp cut-off frequency characteristics above and below said frequencyband. A further object of the invention is to provide an improvedultra-high-frequency band pass filter having a balanced output circuitwhich is push-pull resonant to the mean frequencyy of the desiredfrequency pass band. A still further object of the invention is toprovide an improved ultra high frequency band pass filter comprising aplurality of cylindrical inductive elements having series capacitivetuning means and arranged in parallel spaced relation within a commonshielding enclosure. An additional object to provide an improved`ultra-hisii-frequencyI band pass filter network having a plurality ofspaced self-resonant inductive elements including adjustable series andparallel capacitive elements. and including a balanced output circuitcomprising, two, of such resonant inductive elements exhibitingpush-pull resonance to the mid-frequency of the desired frequency passband.

The invention will be described in greater detail by reference to theaccompanying draw-A ing of which Figure 1 is a schematic circuit diagramof a portion of a typical irequency modulation radar system employingthe invention, rigure Zis. a graph. illustrative of. the,v trans.-mission characteristics. of' the. filter networli. em.- ployect in the.circuit. of Figl., Figure. 3: is a. schematic circuit diagram or' asecond embodiment of the invention, Figure 4 is, an elevationalcross-sectional View of' a. thirds embodiment ol the invention, and.Figure 5' is a topf crosssectional View ol'- sa-id third embodiment.oi'v the invention. Similar reference characters are applied to similarelements throughout. the drawi Referring to Figure l ofthe drawing,the.- invention isdescribedwith reference to its application as a sideband nlter in a frequency modulation object locator system. Afrequencymodulated radar transmitterconnected to a transmission antenna3 generatesfrequencymodul-atedV continuous Wave signals having a meanfrequency, for example. of 5115 mc., thev modulation side bandsextending from 508 to 522 mc. A portion or the transmitted frequencymodulated energy is coupled through anultrahigh-frequency transformer 5to the cathode circuit of a triodeA oscillator-modulator tube Theoscillator-modulatorA tube' l has its grid electrode 9? grounded. Theanode H' is connected'. through a. radio frequency choke coilA It" to aresonant circuit comprising an inductor Iii` and shunt capacitor, litunedto a frequency of mc. A source of. anode. potential, ncitshown,y isserially connected through thev inductor 1.15, and radio frequency chokecoilv la., to` the. anode. I,I of the tube..k A bypass, capacitor L9.is, connected between the source ofv anodepotental and ground, The 30mc. resonant.circuity l5.,A l1' is, inductively Coupled to a. cathodeinductor 2l. which. is con.- netted4 through. the.. Secondary Winding2.3. of. the transformer 5 to. the;y cathodeV` 25 of the. tube. The lowpotential terminal 2T of.tl1e..secondary winding: 23 of the transformer5.v isl coupled,y to ground through. a cathode coupling capacitor 29.The low potential terminal`A 3l. of the cath` odeinductor 2-I isgrounded through a: shunt.- connected resistor 33 and. capacitor 35.

The circuit thus describedl generates 30; mc. local oscillations whichare. modulated in; the tubev l with the 5.1;5. me. frequency-modulatedsignalscoupled thereto` through;V the. ultra-hightrequency transformer5, The-'radioI frequency Choke y coil I?.y preventsloading of-, thelter; net.- workinputpcircuit-by the 30.".irncoscillator; tank circuitI5, l'l.4 The anode H.; of the tube 1: is

coupled; through. an. outputT capacitor 31;Y to; a:v

selected.l pointl 3.9 on.y a.. first; cylindrical.; resonant inductiveelement 4i of the side band filter rietworli c3 constituting the instantinvention.

l'he iilter networx e3 includes resonant inductive elements 4|, 45, liland is disposed in spaced parallel relation within a shielding enclosure5|. The inductive element IH is tuned by an adjustable series capacitorand a shunt capacitor 55. The inductive; element.. d5. is tuned by anadjustable series capacitor 5l and a shunt capacitor 59. The inductiveelements 41 and 4.9,. disposed adjacent to the end or" the shieldingenclosure 5l remote from the input connection e9, are simultaneouslytuned by a common Series. capacitor lil and by separate shunt capacitors53, 65, respectively. The adjustable series capacitors 53, 5J! and 6lare adjusted by rotation of threaded cylindrical members 'l, 69, llthreaded into the top wall of the shielding enclosure 5l. The firstresonant circuit lll, 53, and the second resonant circuit 45, 5l, 59 ofthe filter network. da are. tuned. to.A resonance at. the desired passmidffrequency ofv mc. which is the differencel frequency between themean transmitted 5.1-1.5 rnc. frequency-modulated signals. and thelocally generated. 30.r mc. oscilla--` tions. The pass frequency band ofthe filter is indicated in the graph of. Figure- 21. The outputtunedcircuit. comprising. the elements 47,. 4.5:, 6l, (i3v and 5.5 is.tuned` to push-pull resonance at. the., meanpass frequency of 4.85.:mc., thepush.-v push. resonance thereof. being welL removed: from the;pass frequency bandi.v

QutputA connections. from. predetermined points; i3.. 15. on the.output; inductive; elements; 411,. 4S, respectively, are connectedthrough series re-v Sistors. 1l... '19,.v respectively.,. to a balancedfirstr detector 8l of the frequency-modulation. radarl receivingsystem..The balanced detector. 8l isy also connected to a receiving antenna 83'.for: reception. of. frequency modulated 515 mo. signaisV derived byreflection from an. ob-jectv to. be7 detected.. A balanced first'.detector 8l. is enrployed in. the.v receiving circuit to. eliminate.receiver. response. to. amplitude. modulation. of ther transmitted` or;received` signals.. The difference frequency modulation; component; offthe. balanceddetector 8 l; comprising.A 30.- m-c. signals` representingthe. difference frequency between. the received 515z mc.. andi the. 4851mc. signals. de:- rived from. the;y lter network 43: is coupledthroughs, 30. mc.. intermediate-frequency amiplier 85,110.2. seconddetector SJ; In thezsec-r ond; detector, the intermediate" frequency:sig'.-` nals. are comparedy with the. locally. generated 30.

. mc. signals which. are coupled. to thesecond de'-l tector from anintermediate point 89T on the local oscillatori inductor t5 throughv a.coupling capacitorA 91. The outputzsignals. derived from. the. secondydetector 8l. Yare applied. to` an. ama plifier 93 and utilized in anydesiredi manner: to. provide the required information concerning the.objects toI be detected;

It` is essential. that` the 485`v mc. mixing sig. nals derived from.thelilter network 431contairrai minimum of 515;. 545 and 30. mc.Vmodulation components. The. presence ci a` strong modulation componentiof.: 51'5imc. in'. the mixing sig,- nals will produce. a. relativelylarge.; 30. mc., volt..- age. in the.. first: detector: even. though. nosignalis receivedl on the receiving antenna. This would causemicrophonicsf in. the intermediate frequency. amplifier tube. and-1would. also reproduce; any existing transmit-ter. amplitudesmodm.lation. The novel. side bandi iilter describedi heretofore inconjunction with .the balanced. first 5 detector operation serves toreduce this ,undesired feedthrough to negligible proportions. Theresistors 11, 19 in the coupling circuit from the filter to the balanceddetector are utilized to insure low Q and wide frequency band responsein the detector circuit.

The function of the balanced detector is to heterodyne the reflectedfrequency v.modulated signals derived from the object to be detectedwith the signal derived from the output of the side band filter. Byvirtue of its balanced circuit, the first detector will discriminateagainst amplitude modulated signals received from both the receivingantenna or generated in the transmitter or side band filter circuits,and will provide additional discrimination against the effects of allbut the desired lower side band mixing signals. Since the receivedsignal circuit and the heterodyning circuits are tuned to frequenciesseparated by 30 mc., there is little interaction between them withconsequent less critical balance than in conventional straight audiosystems.

In order to tune the lter network 43 to provide the desired frequencyband width response, it is essential that each of the coupled circuitsbe tuned to resonance to the mean pass frequency of the desired sideband and that the coupling between adjacent inductive filter elements beadjusted to provide the desired band width. Such coupling adjustment maybe provided by any convenient mechanical means whereby the physicalseparation of the inductive elements may be adjusted. For example, theinductive elements may be secured to the shielding enclosure 5| byscrews through longitudinal slots 95 in the enclosure base.

A second embodiment of the invention illustrated in Figure 3 is similarin all respects to that described heretofore by reference to Fig. 1 withthe exception that adjustable shielding elements |03 extending into theshielding enclosure 5| intermediate the inductive elements 4|, 45 and45, 41, 49 permit variation of coupling between successive filternetwork sections by adjustment without the enclosure of the penetrationof the shielding elements within the shielding enclosure. The shieldingelements and ID3 are supported within the shielding enclosure 5| byinwardly and outwardly extending contact elements |95 mounted adjacentto entrance apertures in the top wall of the shielding enclosure.

Another feature wherein the device of Fig. 3 differs from that of Fig. 1is that a single output inductive element 41 is coupled to the balanceddetector circuit by means oi a coupling loop |01 having its center pointgrounded to the shielding enclosure. The coupling loop |01 is coupledthrough a transmission line |99 to the balanced detector 8| forheterodyning the received frequency-modulated signals. Unless theoperating frequency is relatively high, such a coupling loop generallyprovides insufficient coupling to the balanced detector circuit sincethe loop circuit must necessarily be of low Q for ease of alignment andcircuit stability. Therefore, the output coupling circuit shown in Fig.1 is generally to be preferred even though the output coupling resistors11, 19 dissipate some of the filter output energy.

The third embodiment of the invention illustrated in Figures 4 and 5 ofthe drawing is similar to the filter network illustrated in Fig. 1 withthe exception that the series iilter tuning capacitors 53, 51, 6| areentirely enclosed within the shielding enclosure 5|, comprisingcylindrical elements 51, 69 and 1| threaded into collars Ill, ||3 and Il5, respectively. Locking screws i l1 are pro-r vided for each of thecollars. Adjustment of the series capacitors 53, 51, 6| is permittedthrough apertures I I9 in the top wall of the shielding enclosurealigned with each of the threaded cylindrical elements.

Another distinguishing feature of said third embodiment of the inventionis that the shunt tuning capacitors 55. 59, 63 and 65 comprisecylindrical conductive rods extending transversely within the shieldingenclosure and disposed adjacent to and perpendicular to the cylindricalinductive elements 4|, 45, 41, 49, respectively. By either vertical orhorizontal adjustment of the positions of the capacitive rods 55, 59, 63and 65 with respect to their associated inductive elements, thecapacitance therebetween may be readily adjusted, and the coupling tothe inductive elements may be selected at any desired point thereon. Athird distinguishing feature of the third embodiment of the invention isthat only the coupling between the second and third filter sections isadjustable by adjustment of the position of a shielding element |2|disposed between the inductive element 45 and the output inductiveelements 41, 49.

It should be understood that, in any of the embodiments of the inventiondescribed heretofore, the number of filter sections to be employed willdepend upon the desired selectivity and sharpness of frequency cut-offrequired. Three filter sections have been employed in each of theembodiments of the invention purely by way of illustration.

Thus the invention described comprises an improved ultra-high-frequencyband-pass filter network employing a plurality of cylindrical inductiveelements combined with individual series and shunt capacitive tuningelements and all enclosed within a common shielding enclosure. Thecoupling between each of the filter sections is adjustable by adjustmentof the spacing or shielding therebetween for controlling the selectivityandl frequency cut-off characteristics of the system. Externaladjustments are provided for tuning eachof the lter sections. A balancedoutput circuit is provided for coupling the filter to a balanced loadcircuit such as a balanced ultrahigh-frequency detector.

I claim as my invention:

l. A microwave band-pass filter network comprising a hollow shieldingenclosure, a pluralityy of cylindrical inductive elements disposed inmutually inductively coupled spaced relation within said enclosure,separate adjustable capacitive elements interposed between said induc-ltive elements and a wall of said enclosure for tuning said inductiveelements, the relative tuning of said elements and the mutual couplingtherebetween being determinative of the frequenoy band-passcharacteristics of said network, input signal means extending into saidenclosure and coupled to one of said inductive elements, and balancedoutput signal means extending into said enclosure and coupled to twoother adjacent ones ofsaid inductive elements.

2. A microwave band-pass filter network comprising a hollow shieldingenclosure, a plurality of cylindrical inductive elements disposed inmutually adjustably inductively coup-led parallel spaced relation withinsaid enclosure, separate adjustable capacity elements seriallyinterposed between said inductive elements and a wall of said enclosurefor separately tuning each of said inductive elements, the relativetuning of said ele mentsv and the mutualV coupling therebetween beingdeterminative of. the frequency band-pass characteristics. of saidnetwork, two .of said adjacent inductive elements being push-pullreionant at an operating pass frequency oi' said network, input signalmeans extendinginto said enclosure .and coupled to one of. saidinductive elements, and balanced output signal means ex tending. intosaid enclosure and coupled to said two adjacent inductive elements.

3. A microwave band-pass filter network comprising a hollow shieldingenclosure, a plurality of cylindrical inductive elements disposed inmu,- tually adjustably inductively coupled parallel spaced relationwithin said enclosure, separate adjustable capacitive elements seriallyinterposed between said inductive elements and a wall of. said enclosurefor. separately tuning each of said inductivev elements, the relativetuning of said elements and the mutual coupling therebetween beingdeterminative of the frequency band-pass characteristics oi saidnetwork, two of said adjacent inductive. elements being push-pullresonantA at an operating pass frequency or said network, additionalcylindrical fixed capacitive elements projecting from the walls or saidenclosure and extending therein closely adjacent to and perpendicularlydisposed with respectV to said inductive elements, input signal meansextending into said enclosure and coupled to one of said inductiveelements, and balanced outputV signal means extending into saidenclosure and coupled to said. two adjacent inductive elements.

4. A microwave band-pass filter network comprising a hollow rectangularshielding enclosure, a` plurality of cylindrical inductive elementsdisposed. in mutually adjustably inductively coupled parallel. spacedrelation within said enclosure, separate adjustable capacitive elementsserially interposed between said inductive elements and a wall of. said.enclosure for separately tuning each of said inductive elements, therelative tuning o said. elements and the mutual coupling 'therebetweenbeing determinative of the. frequency band-pass characteristics of said.network, two adjacent ones of said inductive elements being push-pullresonant to an operating pass frequency'of. said network, additionalcylindrical. fixed capacitive elements rejecting. from the walls oi saidenclosure and extending therein closely adjacent to and perpendicularlydisposed with respect to said inductive elements, input signal meansextending into said enclosure and connected to one of said inductiveelements, and balanced output signal means extending into said enclosureand coupled to said two adjacent inductive elements.

5. A microwave band-pass lter network comprising a hollow shieldingenclosure, a plurality of cylindrical inductive elements disposed inmutually adjustably inductively coupled parallel spaced relation withinsaid enclosure, means for adjusting the relative spacing of saidinductive elements, separate adjustable capacitive elements seriallyinterposed between one end of said inductive elements and a wall of saidenclosure for separately tuning each ci said inductive elements, therelative tuning of. said elements and the mutual coupling therebetweenbeing determinative of the frequency band-pass characteristics ofsaid'network, one of said adjustable capacitive elements being seriallycoupled to two adjacent inductive elements and providing push-pullresonance thereof at an operating pass frequency, additional cylindricalfixed capacitive elements projecting from the walls of said enclosureand ex'i tending therein closely adjacent to andv perpendlculariydisposed with respect to said inductive elements, input signal meansextending into said enclosure and coupled. to one or said inductiveelements, and balanced output signal .means extending into saidenclosure and coupled to said two adjacent inductive elements.

A microwave band-pass nlter network comprising a hollow shieldingenclosure, a plurality of. cylindrical inductive elements disposed 1nmutually adjustacly inductively coupled parallel spaced relation withinsaid enclosure, separate, adjustable capacitive elements seriallyinterposed between said inductive elements and a wall of said enclosureior separately tuning each oi' said inductive elements, tne relativetuning oi' said elements and tne mutual coupling therebetween beingdeterminative oi' the irequency band-pass characteristics of saidnetwork, one oi said ad justable capacitive elements being seriallycoupled to two adjacent inductive elements and providing push-pullresonance thereof at the operating mean pass irequency, additionalcylindrical nxed capacitive elements, projecting from the walls of saidenclosure and extending therein closely adjacent to and perpendicularlydisposed. with respect to said inductive elements, iiist signal meansextending into said. enclosure and con-` nected to an intermediate pointon one ci' said inductive elements, and balanced second signalconnections extending into said enclosure and separately connected tosaid two adjacent inductive elements.

7. .A microwave pand-pass filter network comprising a hollow shieldingenclosure, a pluralityv of cylindrical inductive elements disposed inmutually adjustably inductively coupled parallel spaced relation withinsaid enclosure, separate adjustable capacitive elements seriallyinterposed between said inductive elements and a wall of said enclosurefor separately tuning each of sad in1 ductive elements, the relativetuning of said ele-- ments and the mutual coupling therebetween being.determinative of the irequency band-passvv characteristics of saidnetwork, additional cylin drical xed capacitive elements projecting tromthe walls of said enclosure and extendng therein closely adjacent to andperpendicularly disposed with respect to said inductive elements, firstsignal means extending into said enclosure and connected to one of saidinductive elements, and balanced second signal means comprising acoupling loop extending into said enclosure and coupled to one of saidinductive element-s.

8. A microwave band-pass filter network comprising a hollow rectangularshielding enclosure, a plurality of cylindrical inductive elements dis'-posed in mutually adjustably inductively coupled parallel spacedrelation within said enclosure par.- allel to one of the axes thereof,separate ad justable capacitive tuning elements serially inter-l posedbetween said inductive elements and a wall of said enclosure forseparately tuning each of said inductive elements, the relative tuning'of said elements and the mutual coupling there'v between beingdeterminat've of the frequency band-pass characteristics of saidnetwork, one of said adjustable capacitive elements being seriallycoupled to two adjacent inductive elements lccated. adjacent to one endof said enclosure and providing push-pull resonance thereof at the de-Vsired mean-pass frequency, additional cylindrical fixed capacitivetuning elements projecting from the. walls of said enclosure andextending therein closely adjacent to and perpendicularly disposed withrespectl to said inductive elements, input signal means extending intoand adjacent to one end of said enclosure and coupled to one of saidinductive elements, and balanced output signal means extending into andadjacent to the opposite end of said enclosure and coupled to said twoadjacent inductive elements.

9. Apparatus according to claim 8 including means for adjusting therelative spacing of said inductive elements.

10. Apparatus according to claim 1 including means for adjusting therelative spacing of said inductive elements.

11. A microwave band-pass filter network comprising a hollow rectangularshielding enclosure, a plurality of cylindrical inductive elementsdisposed in mutually adjustably inductively coupled parallel spacedrelation within said enclosure in a plane parallel to one of the axesthereof, means for adjusting the relative spacing of said inductiveelements in said plane, separate adjustable capacitive tuning elementsaxially interposed between one end of said inductive elements and a wallof said enclosure for separately tuning each of said inductive elements,the relative tuning of said elements and the mutual couplingtherebetween being determinative of the frequency band-passcharacteristics of said network, one of said adjustable capacitiveelements being serially coupled to two adjacent inductive elements1ocated adjacent to one end of said enclosure and providing push-pullresonance thereof at the desired mean-pass frequency, additionalcylindrical xed capacitive tuning elements projecting from the walls ofsaid enclosure and extending therein closely adjacent to andperpendicularly disposed with respect to said inductive elements, inputsignal means extending into and adjacent to one end of said enclosureand connected to one of said inductive elements, and balanced outputsignal means extending into and adjacent to the opposite end of saidenclosure and coupled in opposite phase to said two adjacent inductiveelements.

HARRY KIHN.

REFERENCES CITED The following references are of record in the le ofthis patent:

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